Constrictive Deformation in Ice: A Glaciological Approach

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It is impossible to formulate accurate models of Earth’s climate without an understanding of ice flow in glaciers and on ice shelves, where it is undergoing constrictive deformation. The larger scale dynamics of glacier flow have been well-studied, its microstructure is less examined. The aim of this study is to examine the creep and microstructural behaviour of ice undergoing constrictive deformation, and compare it to real glacial flow.
A series of experiments were performed by forcing cylinders of laboratory ice through a narrowing aluminium cylinder at -5.5?C, while measuring displacement. Electron Backscatter Diffraction (EBSD) maps were produced of several samples to allow their microstructure to be studied. In addition, images from the Mosaic of Antarctica (MOA) satellite map were used to perform a radial strain analysis on Byrd Glacier, Antarctica, to quantify radial strain, radial strain rate and ice mass flux down its length.
Experimental results suggest that ice undergoing constrictive deformation experiences localised deformation at grain boundaries during primary creep, which becomes homogeneous during secondary creep if the sample has not experienced fracture in the early stages of deformation. Where fractures are present, deformation tends to remain heterogeneous. The accelerating stage of creep is approached by fractured samples, but is not definitively reached by any samples in this experimental series.
Mechanical behaviour observed in the experimental dataset is broadly similar to that commonly observed in uniaxial experiments for ice which is initially anisotropic. The mechanical behaviour observed in the Byrd Glacier dataset is extremely similar to that observed in uniaxial experiments on ice which has a starting CPO oriented for easy slip in that stress regime. This supports the theory that ice entering Byrd Glacier has a vertical girdle CPO oriented perpendicular to the direction of flow, an easy-slip arrangement for constrictive flow, and so may bypass secondary creep when it enters the glacier and is subjected to high radial strains. As a result, it is unlikely to experience a concentration of stress at grain boundaries. However, small variations in topography and basal lubrication are likely to cause stress heterogeneities which are not accounted for in current models. Experimental results suggest that these heterogeneities cause deformation to become preferentially localised, which can cause thickness-scale folding.